Gramicidin A aggregation in supported gel state phosphatidylcholine bilayers

Area of Science:

• Biophysics
• Materials Science
• Membrane Biology

Background:

• Gramicidin A (gA) is a channel-forming peptide incorporated into lipid bilayers.
• Understanding gA aggregation is crucial for elucidating its function in biological membranes.
• Previous studies lacked direct visualization of gA aggregation in supported bilayers under physiological conditions.

Purpose of the Study:

• To directly visualize and characterize the aggregation of gramicidin A (gA) in supported saturated phosphatidylcholine bilayers.
• To investigate the influence of gA concentration and lipid acyl chain length on aggregation patterns.
• To identify the fundamental aggregation unit of gA within lipid membranes.

Main Methods:

• Atomic Force Microscopy (AFM) was employed to image supported bilayers.
• Experiments were conducted in aqueous solutions at room temperature.
• Supported bilayers consisted of saturated phosphatidylcholine with varying concentrations of gramicidin A.

Main Results:

• Gramicidin A (gA) directly formed observable clusters in gel-state phosphatidylcholine bilayers.
• At low concentrations, gA formed small clusters (likely hexamers) dispersed within lipids, forming elongated aggregates but not pure domains.
• Increased gA concentration led to larger domains or interconnected networks (percolation transition) in shorter-chain lipids, while longer-chain lipids formed lamellar structures with ripple morphology.

Conclusions:

• The fundamental aggregation unit of gramicidin A (gA) appears to be a hexamer, consistent across lipid acyl chain lengths of 14-18 carbons.
• Gramicidin A aggregation transitions from dispersed clusters to connected networks or complex lamellar structures based on concentration and lipid chain length.
• The molecular mechanisms underlying the formation of observed lamellar and ripple structures require further investigation.